(1) Field of the Invention
The present invention relates to a method for forming an on-chip lens and a method for manufacturing a solid-state imaging apparatus, and particularly to a technique for forming a depression by using an etch-back method.
(2) Description of the Related Art
Solid-state imaging apparatuses are widely used as image input apparatuses for a portable apparatus and a camera. Now, there is a demand for further miniaturizing the solid-state imaging apparatuses and increasing the number of the pixels thereof. Therefore, the solid-state imaging apparatuses have a structure that includes, in addition to the top lenses that have been conventionally provided, lenses (in-layer lens) nearer to the photoelectric converter unit (photodiode) than the top lenses are. In other words, for realizing a solid-state imaging apparatus having small pixels, it is necessary to form an on-chip lens including an in-layer lens in terms of the sensitivity and the smear characteristics.
Regarding the in-layer lens of a solid-state imaging apparatus, method for forming the in-layer lens is suggested by various documents (Japanese Laid-open Patent Application Publication No. 2002-246578, Japanese Laid-open Patent Application Publication No. H06-61487, Japanese Patent Publication No. 2597037, and Japanese Laid-open Patent Application Publication No. 2002-353428).
For example, the following describes a structure of a solid-state imaging apparatus in which the in-layer lenses are formed by the method suggested by Japanese Laid-open Patent Application Publication No. 2002-246578, with reference to FIG. 1A and FIG. 1B. Note that only the imaging area of the solid-state imaging apparatus is illustrated in FIG. 1A and FIG. 1B.
As FIG. 1A shows, the solid-state imaging apparatus has a gate insulation film 1014 formed on one of main surfaces of a semiconductor substrate 1011, and transfer electrodes 1015 are formed at intervals on the gate insulation film 1014. Over the transfer electrodes 1015, an interlayer insulation film 1016, a light shielding film and a passivation film 1018 are formed in this order. On the passivation film 1018, an in-layer lens first formation film 1019 and an in-layer lens second formation film 1020 are formed, and a color filter film 1021 and a top lens formation film 1022 are also formed. Moreover, in the area extending inwardly from the main surface of the semiconductor substrate 1011, a pair of charge transfer channel units 1012, and a photoelectric converter unit 1013 located therebetween are formed.
The top lens formation film 1022 is in a convex shape where a portion corresponding to the photoelectric converter unit 1013 is upheaved. This upheaved portion is a top lens 1022a, which is formed for each pixel. On the other hand, the in-layer lens first formation film 1019 is in a concave shape where a portion corresponding to the photoelectric converter unit 1013 is depressed. This depressed portion is embedded by the in-layer lens second formation film 1020, and this forms an in-layer lens 1020a. 
In Japanese Laid-open Patent Application Publication No. 2002-246578, a film made of a BPSG (Boron Phosphorous Silicon Glass) is formed along the surface of the passivation film 1018. Such a BPSG film is processed by the heat-flow method. As a result, the in-layer lens first formation film 1019, in which the portion corresponding to the photoelectric converter unit 1013 is depressed, is formed. However, in the case of forming the in-layer lens by the heat-flow method, an inflection point PB is formed at the depression of a boundary L1019 between the in-layer lens first formation film 1019 and the in-layer lens second formation film 1020, as FIG. 1B shows. Therefore, if the technique disclosed by Japanese Laid-open Patent Application Publication No. 2002-246578 is applied, only an in-layer having a small effective diameter can be formed. This should be improved, from the viewpoint of improving the light concentration efficiency.
Japanese Patent Publication No. 2597037 suggests a technique to form a mask having a fine opening at a portion corresponding to the photoelectric converter unit, and form a depression having a sector-shaped cross section through the opening, by using the isotropic etching to form the lens.